Calibration methods and circuits for optimized on-die termination

ABSTRACT

Described are on-die termination (ODT) systems and methods that facilitate high-speed communication between a driver die and a receiver die interconnected via one or more signal transmission lines. An ODT control system in accordance with one embodiment calibrates and maintains termination resistances and drive currents to produce optimal output swing voltages. Comparison circuitry employed to calibrate the reference resistance is also used to calibrate the drive current. Termination elements in some embodiments are divided into two adjustable resistive portions, both of which are designed to minimize capacitive loading. One portion is optimized to produce a relatively high range of adjustment, while the other is optimized for fine-tuning and glitch-free switching.

BACKGROUND

High-speed data communication integrated circuit (IC) dies are known toinclude both drivers and receivers. The driver of one such IC connectsto the receiver of another via one or more signal transmission lines.Both the driver and receiver circuits include termination elements thatattempt to match the characteristic impedance of the transmission lineto the output impedance of the driver and input impedance of thereceiver, as impedance mismatches degrade signal quality andconsequently reduce communication speed and reliability.

Some conventional communication systems employ control systems thatcalibrate the impedance of on-chip termination elements for improvedimpedance matching. For a detailed discussion of one such system, seeU.S. Pat. No. 6,418,500 entitled “Feedback Control for TerminationAdjustment,” which issued on Jul. 9, 2002, to Gai et al.

Some high-performance communication systems employ current-modeswitching for improved speed performance. Such systems includecurrent-mode drivers, which convey digital signals by modulating currentbetween values representative of different logic levels. The levels ofcurrent used to express logic levels are carefully calibrated andcontrolled to obtain rapid switching and low power consumption. Thefollowing issued patents describe conventional circuits for calibratingthe drive current for current-mode drivers: U.S. Pat. No. 5,254,883entitled “Electrical Current Source Circuitry for a Bus,” which issuedon Oct. 19, 1993, to Horowitz et al., and U.S. Pat. No. 6,462,591entitled “Semiconductor Memory Device Having a Controlled Output DriverCharacteristic,” which issued on Oct. 8, 2002, to Garrett et al.

The aforementioned calibration techniques for setting and maintainingappropriate termination resistances and drive currents work well in manyapplications. High-speed circuits are in a very competitive market,however, and must achieve ever greater performance levels to satisfycustomer demand. There is therefore a need for area- and power-efficientcommunication circuits that exhibit improved speed performance.

SUMMARY

The present invention is directed to efficient on-die termination (ODT)systems and methods that facilitate high-speed communication between adriver die and a receiver die interconnected via one or more signaltransmission lines. An ODT control system in accordance with oneembodiment calibrates and maintains the termination resistances anddrive currents of the driver die to produce optimal output swingvoltages on the signal transmission lines. A similar ODT control systemcalibrates and maintains the termination resistances of the receiverdie. The ODT control systems rely upon external voltage and resistancereferences for termination resistance and drive current calibration.

An ODT control system in accordance with one embodiment calibrates drivecurrent by matching the drive current to a reference current drawnthrough a precision resistor, and calibrates termination resistance bymatching on-die termination elements with the same external resistor.Comparison circuitry employed to match the reference and drive currentsis also used to match the reference and ODT resistances. This sharing ofresources saves power and die area. In addition, offset errorsintroduced by shared components impact the resistance and currentcalibrations in opposite ways, and thus tend to cancel one another.

Termination elements in some embodiments are divided into two adjustableresistive portions, both of which are designed to minimize capacitiveloading. One portion is optimized to produce a relatively high range ofadjustment, while the other is optimized for fine-tuning and glitch-freeswitching. Active ODT control systems can calibrate the first portionbefore respective drive and receive circuitry begin communicating, andcan then employ the second portion to account for resistive drift thatoccurs due to supply-voltage and temperature fluctuations. In oneembodiment, the second portion employs a serial, thermometer-codedvoltage divider to provide glitch-free switching between ODT resistancelevels. In another embodiment, the second portion employs an analogvoltage divider. Either embodiment can be used to periodically calibrateODT resistance without introducing undesirable resistive glitches.

This summary does not limit the invention, which is instead defined bythe allowed claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a data communication system 100 inaccordance with one embodiment.

FIG. 2 depicts a termination resistance and drive current calibrationsystem 200 that includes ODT control system 130, reference resistor 175,and reference source 170 of FIG. 1.

FIG. 3 depicts a process 300 employed by control system 130, as detailedin FIG. 2, to calibrate reference ODT resistor 230 and ODT current Iodt.

FIG. 4 depicts a communication system 400 in accordance with anotherembodiment.

FIG. 5 details an embodiment of calibration system 200 of FIG. 2.

FIG. 6 depicts an ODT termination element 600 in accordance with oneembodiment.

FIG. 7 depicts a termination element 700 in accordance with anotherembodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a data communication system 100 inaccordance with one embodiment. Communication system 100 includes afirst integrated circuit (IC) die 105 connected to a second IC die 110via a number of signal transmission lines 115. Die 105 includes aplurality of identical driver circuits 120 and an on-die-termination(ODT) control system 130. Control system 130 establishes and maintainsthe termination resistances and drive currents of driver circuits 120 toproduce optimal output swing voltages.

Die 110 includes a plurality of identical receiver circuits 125, each ofwhich receives signals from a corresponding one of driver circuits 120of die 105. Die 110 includes an ODT control system 187 that establishesand maintains the termination resistance applied to incoming signals tooptimize the swing voltages of those signals.

Each of dies 105 and 110 may be any of myriad types of processing chipscapable of communicating electrical signals. Typical examples include ICdies that communicate via parallel or serial bus interfaces.Transmission lines 115 may be, for example, a wire, a cable, a trace ona printed-circuit board, a chip etching, a waveguide, or other mediumthat allows for transmission of signals.

For simplicity, die 105 and die 110 are presumed to support respectivedriver and receive circuitry; in other embodiments, however, die 105additionally supports receive circuitry, and die 110 additionallysupports transmit circuitry. Such embodiments can use eitherunidirectional or bidirectional signal lines, as is well known to thoseof skill in the art. Also important, the depicted embodiment isdescribed in connection with a typical case in which two diescommunicate signals via external lines, but other embodiments optimizesignal line termination resistance, drive current, and signal swing toimprove communication speed between circuits that exist on the same die.Driver circuits 120 are identical as are receiver circuits 125. Thefollowing description is limited to the topmost driver/receiver pair forbrevity.

Driver circuit 120 includes a driver 135 receiving an input data streamrepresented as a pair of complimentary input signals Din0 and Din0 b,which are provided on complimentary driver input nodes having the samedesignations. (As with other designations herein, Din0 and Din0 b referboth to signals and their corresponding nodes; whether a givendesignation refers to a signal or a node will be clear from thecontext.) Driver 135 additionally includes a pair of driver outputterminals 150 and 155 that connect to signal transmission lines 115 viaIC pads (not shown). Driver output terminals 150 and 155 conveycomplimentary output signals derived from the input data stream. (Unlessotherwise defined, terminals, lines, conductors, and traces that carry agiven signal fall under the umbrella term “node”; in general, the choiceof a given description of a circuit node is a matter of style, and isnot limiting.)

Driver circuit 120 includes a pair of termination elements 145 and adigital-to-analog converter (DAC) 140. Each of driver output terminals150 and 155 connects to a reference voltage Vio via a respectiveadjustable termination element 145, and each termination element 145receives digital ODT calibration signals from ODT control system 130 viaan ODT calibration port 165. Driver circuit 120 additionally includes acurrent control port 160, which receives digital current-calibrationsignals from control system 130. DAC 140 converts these digital signalsinto a current-control voltage that controls the level of drive currentprovided by driver 135. By controlling both the termination resistanceand the drive current of driver circuit 120, ODT control system 130optimizes the swing voltages on driver output terminals 150 and 155, andconsequently reduces noise and improves speed performance.

The resistance values of termination elements 145 and the drive currentprovided by driver 135 vary with process, voltage, and temperature. ODTcontrol system 130 therefore relies upon precise reference components toestablish appropriate termination resistances and drive currents. In thedepicted embodiment, die 105 is connected to a first reference voltageVr1 through an external voltage source 170 and an external precisionresistor 175. Reference source 170 conveys a second reference voltageVr2 to ODT control system 130. The difference between reference voltageVr1 and Vr2 is used to establish an appropriate swing voltage. Referenceresistor 175 conveys a reference-resistor voltage Vrr to ODT controlsystem 130, acting as a reference resistance against which to compareand adjust termination elements 145. A calibration process forestablishing the appropriate resistance through termination elements 145and drive currents provided by drivers 135 is described below inconnection with FIGS. 2 and 3.

Receiver circuit 125 within die 110 includes a differential amplifier185 connected to a pair of complimentary signal transmission lines 115via respective receiver input terminals 186 and 189. Each of inputterminals 186 and 189 connects to reference voltage Vio via a respectivetermination element 180. Termination elements 180 are similar toelements 145 in this embodiment, and are controlled via a line ODT froman ODT control system 187 similar to system 130. Control system 187compares the values of termination elements 180 with an off-chip,precision reference resistor 195 to account for process, voltage, andtemperature variations that affect the resistance values of terminationelements 180. ODT control system 187 is also capable of calibratingdrive current, though no drivers are shown in FIG. 1. Wheredrive-current calibration is desired, control system 187 connects to anexternal reference voltage 190. Receiver circuit 125 receives theincoming data stream on complimentary input nodes 186 and 189 andpresents a resulting data to the interior of IC die 110 viacorresponding data output node Dout0.

FIG. 2 depicts a termination resistance and drive current calibrationsystem 200 that includes ODT control system 130, reference resistor 175,and reference source 170 of FIG. 1. (In general, the first digit of agiven numerical designation indicates the Figure in which the indicatedelement was introduced. For example, reference resistor 175 wasintroduced in FIG. 1.)

Reference source 170 and reference resistor 175 are external, precisionelements connected to system 130 via a pair of pads 201 and 202. Controlsystem 130 includes a comparator 215, an analog multiplexer 220, areference ODT resistor 230, and a current source 240. Control system 130calibrates the resistance value Rodt of resistor 230 to match thereference resistance Rref of external precision resistor 175. Controlsystem 130 also matches the current Iodt through resistor 230 with areference current Irr through reference resistor 175 to produce adesired voltage drop across ODT resistor 230. ODT counter 245 stores adigital value representative of the offset required to calibrate IDTresistor 230, and a current-control counter 250 stores a digital valuerepresentative of the offset required to calibrate current source 240.The digital values from counters 245 and 250 are then conveyedthroughout die 105 (FIG. 1) to calibrate similar or identicaltermination elements and current sources within driver circuits 120.

FIG. 3 depicts a process 300 employed by control system 130, as detailedin FIG. 2, to calibrate reference ODT resistor 230 and ODT current Iodt.First, at step 305, current-calibration signal ICAL1 b, an active-lowsignal, is asserted. Analog multiplexer 220 thus selects secondreference voltage Vr2 for comparison with reference voltage Vrr fromresistor 175. The inverse of signal ICAL1 b, ICAL1, enablescurrent-control counter 250 to increment and decrement in response tooutput signals from comparator 215.

Current source 240 pulls reference current Irr through referenceresistor 175. Per Ohm's law, reference voltage Vrr equals referencevoltage Vr2 when the product of reference current Irr and referenceresistance Rref equals the swing voltage Vsc across voltage reference170 (i.e., when Irr*Rref=Vsc). Comparator 215 compares voltage Vrr withvoltage Vr2 (decision 310), and adjusts the contents of current-controlcounter 250 as necessary to render voltage Vrr equivalent to voltage Vr2(step 315). For example, if comparator 215 determines that Vrr is lowerthan Vr2, comparator 215 increments counter 250 to reduce current Irr,and consequently increase the voltage Vrr. The comparison and adjustmentof steps 305, 310, and 315 continue until Vrr and Vr2 are substantiallyequal. At this point, current counter 250 stores a count CCNT thatoffsets the current Irr provided by current source 240. ODT current Iodtis substantially equal to reference current Irr, so steps 305, 310, and315 place current Iodt at or near the expected operating level.

The granularity of current-control count CCNT introduces someuncertainty as to the exact reference current Irr and correspondingIodt. Employing more register bits and associated bus lines for CCNTreduces this uncertainty at a cost of increased overhead.

The next portion of calibration process 300 sets resistance Rodt ofresistor 230 substantially equal to reference resistance Rref ofprecision resistor 175 (i.e., Rodt≈Rref) by adjusting the count incounter 245. The first current-calibration signal ICAL1 b is deasserted(held to a logic one), causing multiplexer 220 to apply voltage Vodtfrom resistor 230 to the second input terminal of comparator 215.Deasserting ICALb also disables counter 250, freezing current controlsignal CCNT to hold Iodt and Irr constant. A termination calibrationsignal TCAL to counter 245 is asserted, enabling counter 245 toincrement and decrement in response to output signals from comparator215.

In step 320, comparator 215 compares Vrr and Vodt. ODT counter 245increments or decrements in response to the output of comparator 215,altering the resistance Rodt through ODT element 230. The input voltageVio to termination resistor 230 is connected to off-chip voltagereference Vr1, and is therefore substantially equivalent to voltage Vr1.Because the currents Irr and Iodt are substantially identical, Ohm's lawprovides that voltage Vodt is substantially equal to voltage Vrr ifresistance Rodt of resistor 230 matches reference resistance Rref ofprecision resistor 175. Comparator 215 thus adjusts the contents ofcounter 245 (step 330) until decision 325 determines that voltages Vrrand Vodt are substantially equal. The granularity of ODT-control countODT introduces some uncertainty as to the exact reference resistanceRodt. This uncertainty can be improved by employing more register bitswithin counter 245, but this improvement comes at a cost of increasedoverhead.

Input/output voltage Vio, being distributed on chip, may vary by somesmall amount from external reference voltage Vr1. The resultinguncertainty combines with other uncertainties, such as those resultingfrom the measurement granularities of counters 245 and 250, to producestill greater uncertainty. A second current-calibration sequencerecalibrates ODT current Iodt using the calibrated ODT resistor 230 andinput voltage Vio to reduce this uncertainty.

Returning to FIG. 1, each signal transmission line 115 includes anassociated pair of termination elements 145 and 180 connected inparallel between Vio and respective ends of the signal transmissionline. To duplicate this arrangement, current control system 130 includesa transistor 255 that shorts reference resistor 175 and ODT resistor 230in response to a second current-calibration signal ICAL2, effectivelyconnecting resistors 175 and 230 in parallel between node Vio (recallVr1 is an off-chip version of Vio) and node Vodt. Because voltages Vr1and Vio and resistances Rref and Rodt are substantially equal, theexpected current between interconnected nodes Irr and Iodt is expectedto be quite low.

In step 335, signal TCAL is deasserted and both current-control signalsICAL1 b and ICAL2 are asserted. Counter 245, and thus resistance Rodt,is thus held constant as comparator 215 once again compares Vr2 with Vrr(step 340). Counter 250 increments or decrements in response to outputsignals from comparator 215 (step 350). The calibration process 300 iscomplete when system 130 voltages Vr2 and Vrr are substantially equal(step 345). The resulting offsets held in counters 245 and 250 aredistributed on buses ODT and CCNT to drivers and receivers on the samedie to establish appropriate ODT resistances and drive currents in themanner described above in connection with FIG. 1.

The current and resistance calibration steps are carried out usingshared resources, including the low-offset, high-gain comparator 215.This sharing of resources saves power and die area. In addition, offseterrors introduced by shared components impact the resistance and currentcalibrations in opposite ways, and so tend to cancel one another. Forexample, a comparator error that results in a slightly high ODTresistance Rodt produces a slightly low ODT current Iodt. Rodt and Iodtare multiplied to produce the desired swing voltage, so the increase inRodt tends to cancel the decrease in Iodt.

FIG. 4 depicts a communication system 400 in accordance with anotherembodiment. System 400 includes components in common with communicationcircuit 100 of FIG. 1, like-elements being the same or similar. System400 includes first and second bus interfaces 405 and 410, each of whichis typically a portion of a respective IC die, interconnected viaparallel bus 417. One driver circuit 415 of interface 405 and acorresponding receive circuit 420 of interface 410 are detailed: theremaining driver circuits 415 and receive circuits 420 are representedas simple boxes for brevity.

Driver circuit 415 includes a pair of termination elements 425 (ODTresistors) connected between reference voltage Vio and respective onesof a pair of signal transmission lines. Driver 415 additionally includesa current-controlled amplifier 418, which in turn includes a pair ofinput transistors 430 and a current-source transistor 435. Each oftransistors 430 connects between one of termination elements 425 and apower-supply terminal GND via transistor 435. A DAC 440 within drivercircuit 415 controls the current through transistor 435 by applying acontrol voltage CV to the gate of transistor 435, and consequentlycontrols the maximum voltage drop across termination elements 425.

ODT control system 130 derives offsets ODT and CCNT to calibratetermination resistance and drive current in the manner described abovein connection with FIGS. 2 and 3. DAC 440 derives control voltage CVfrom the current-control offset CCNT so that the drive current ofamplifier 418 equals the ODT current Iodt calibrated in process 300.Termination elements 425 are fabricated to be identical to reference ODTresistor 230, and so respond in kind to offset signal ODT from controlsystem 130. In short, control system 130 calibrates a referencetermination element and a reference drive current by establishingappropriate ODT and CCNT counts, and these counts are applied to drivercircuits 415 on the same die to establish and maintain the correcttermination resistances, drive currents, and voltage swings.

Receive circuit 420 does not generate a drive current, but includes apair of termination elements 445. Like termination elements 425 ofdriver circuit 415, termination elements 445 are calibrated by an offsetODT from the associated ODT control system 187. Though not shown, otherembodiments include interconnected transceivers, each of which includeboth drive and receive circuitry. ODT control system 187 can be used toestablish appropriate termination-element resistance, drive current, andswing voltage levels in such embodiments. Transmission lines can bebidirectional in systems that include interconnected transceiver pairs.

FIG. 5 details an embodiment of calibration system 200 of FIG. 2,like-numbered elements being the same or similar. In this embodiment,ODT control circuit 130 includes additional components, many of whichare included to more closely match the behavior of the referenceelements used for calibration with the termination elements and driversto be calibrated. The example assumes the first reference voltage Vr1and input/output voltage Vio are each 1.8 volts, reference resistanceRref is 50 ohms, swing calibration voltage Vsc is 350 millivolts, andreference current Irr is 7 milliamps; however, these values areillustrative and not limiting.

Terminal Vrr of comparator 215 connects to reference resistor 175 via apass gate 505. Pass gate 505 is included to mirror the behavior of apair of pass gates that are combined to form analog multiplexer 220.Pass gate 505 offers the same resistance as multiplexer 220, and thuscancels voltage differences on the input terminals of comparator 215that would otherwise introduce errors. Complementary outputs fromcomparator 215 feed an ODT counter 506 and current counter 507 via adigital filter 509 and sampling latch 510. Sampling latch 510periodically samples the output of filter 509. If enabled, one ofcounters 506 and 507 increments or decrements in response to the outputfrom sample latch 510 to adjust the count on one of buses ODT<8:0> orCCNT<6:0>. Comparator 215, filter 509, and latch 510 are shared to savepower, and die area, and to reduce offset errors in the manner discussedabove in connection with FIG. 2.

Current source 240 includes a resistor 515, a reference current source520, a DAC 525, and a pair of current-source transistors 530. Resistor515 is a replica of ODT resistor 230, but may be of a different size orvalue. Resistor 515 is formed using the same process sequence asresistor 230, and so varies in the same manner with process, voltage,and temperature. Current source 520 produces a reference current Irefproportional to the value of resistor 515. DAC 525 uses referencecurrent Iref to derive a control voltage CCREF proportional to theresistance of resistor 515, and consequently proportional to Rodt.Current-control bus CCNT<6:0> then provides offset values to DAC 525 toadjust control voltage CCREF as necessary to calibrate currents Irr andIodt. In one embodiment, current source 240 provides a range of outputcurrents Irr and Iodt that range from about zero to about twice Iref.Current source 240 begins the calibration sequence in the middle of thisrange (Irr=Iodt=Iref).

Current source 240 includes an additional pair of transistors 535 and540, which serve two purposes: first, transistors 535 and 540 candisable current source 240 to save power when ODT control system 130 isnot in use; second, when ODT enable signal ODTEN is asserted,transistors 535 and 540 combined with transistors 530 emulate thebehavior of transistors 430 and 435 within driver 415 of FIG. 4.Transistors 535 and 540 are the same size (W/L) as transistors 430, andtransistors 530 combined are the same size as transistor 435. Withcurrent calibration signal ICAL2 asserted to forward bias a pair oftransistors 545 and 550, transistors 530, 535, and 540 and reference ODTresistor 230 and reference resistor 175 together emulate the behavior oftransistors 430 and 435 and termination elements 425 of driver circuit415. The counts within counters 506 and 507 employed to establishappropriate termination resistances and drive currents within system 130are therefore representative of the counts required to properly adjustthe termination elements and drive currents depicted in FIG. 4.

FIG. 6 depicts an ODT termination element 600 in accordance with oneembodiment. Termination element 600 can be used in place of ODT resistor230 of FIGS. 2 and 5 and in place of termination elements 425 and 445 ofFIG. 4.

Termination element 600 includes a number of OR gates, each of whichincludes one input terminal connected to a high-impedance-selectterminal RHIZ. When asserted, signal RHIZ disables termination element600, providing a high impedance between reference voltage Vio and theassociated transmission line (for termination elements 425 and 445) orto multiplexer 220 (for reference ODT resistor 230). The followingdescription assumes signal RHIZ is deasserted, so the OR gates merelypass whatever signal is provided on their respective second inputterminals.

Termination element 600 includes two separate portions: abinary-weighted portion 605 and a thermometer-coded portion 610. Portion605 includes a resistor network 635 and a collection of PMOS transistors630, the gates of which are connected to the first six lines ODT<5:0> ofcontrol bus ODT<8:0> of FIG. 5. Each PMOS transistor 630 controls thecurrent through a respective resistive path. These resistive pathsprovide binary-weighted resistances from about 50 to 1600 ohms, soportion 605 can produce a wide range of resistances by enabling selectedtransistors 630. (The weighted resistance values can vary widely withprocess, temperature, and voltage variations, however, hence the needfor ODT calibration.)

Transistors 630 are relatively small to reduce the effect of capacitanceon transmission lines 417, so transistors 630 provide a significantpercentage of the resistance through portion 605 (e.g., as much as about30% in one embodiment). The I-V characteristics transistors 630 aresomewhat non-linear, and this non-linearity produces some non-linearityin the resistance through portion 605. The first current-calibrationsequence sets ODT current Iodt close to the operating current beforeadjusting Rodt so the non-linear response of Rodt does not introducesignificant errors in Rodt when Iodt is adjusted during the swingcalibration. The first current-calibration sequence can be omitted ifaccuracy is less important or if the termination elements exhibit morelinear I-V characteristics.

The binary-weighted scheme of portion 605 provides a relatively lowcapacitance, area-efficient means of producing a large number ofpotential resistance values, but can introduce problematic glitches whenswitching between values. For example, incrementing the six low-orderbits ODT<5:0> from 011111 to 10000 to vary the selected resistance fromabout 50 ohms to about 52 ohms turns off the right-most one oftransistors 630 and turns on the remaining transistors 630. Imperfecttiming can produce intermediate resistance values significantly over orunder the desired 52 ohms. Such a resistive glitch can causetransmission errors in systems that dynamically monitor and adjusttermination values to compensate for supply-voltage and temperaturevariations.

Thermometer-coded portion 610 addresses the glitch problem associatedwith portion 610. Portion 610 facilitates fine resistance adjustments byincrementally connecting transistors 620 in parallel with resistiveelements in a voltage divider 615. Thermometer coding means thattransistors 620 are enabled one at a time, in series. The possiblecombinations of signals on lines CCNT<8:6> are 000 (lowest resistance),100, 110, and 111 (highest resistance). Other schemes are possible, andmore or fewer series resistors can be included. Transistors 620 arearranged in series to minimize the effects of capacitance.

Portion 605 is calibrated in the manner described above and the offsetprovided on ODT<5:0> held steady during operation. Portion 610 canlikewise be held steady, or can be modified continuously or periodicallyto compensate for supply-voltage or temperature fluctuations. Likecontrol bits ODT<5:0>, control bits ODT<8:6> can be derived by a commoncalibration circuit and distributed to a number of termination elements;alternatively, control bits ODT<8:6> can be derived separately for eachsignal pad. An ODT-control signal CNTODTb can be deasserted to disableportion 610, while calibrating portion 605, for example.

Thermometer-coded portion 610 is less area efficient than portion 605,but can be incremented or decremented without introducing undesirableglitches. For further discussion relating to combining thermometer andbinary coding, see U.S. Pat. Nos. 6,606,004 and 6,297,759, each of whichis incorporated herein by reference.

FIG. 7 depicts a termination element 700 in accordance with anotherembodiment. Termination element 700 is similar to termination element600 of FIG. 6, like numbered elements being the same or similar.Termination element 700 differs from termination element 600 in thatthermometer-coded portion 610 of termination element 600 is replacedwith an analog resistive element 705 to facilitate fine-tuning oftermination resistance.

The resistance between terminals Vio and Vodt includes the parallelcontributions from portions 605 and 705. Portion 605 is tuned in themanner described above using an ODT count. An integrator composed of acomparator 710 and capacitor 712 then alters the gate voltage of atransistor 707 to alter the resistance through portion 705 such thatvoltages Vrr and Vodt are equal. The gate voltage on transistor 707 in areference ODT termination element can be distributed to similartransistors 707 in termination elements associated with propagatingsignals. Alternatively, each analog portion 705 can be controlledlocally using a dedicated off-chip reference resistor for each pin. Ineither case, portion 605 of termination elements associated with signalconveyance (e.g., elements 425 and 445) can be controlled by distributedODT control signals ODT<5:0>.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, (1) the externalvoltage and resistance references may be substituted in otherembodiments with on-chip references; (2) the ODT circuits can beconstructed entirely of PMOS transistors, as PMOS transistors behavelike a resistor when operating in the linear region (e.g., with the gateconnected to ground); (3) the thermometer portion 610 (FIG. 6) can havemultiple “legs” connected in parallel; and (4) one or more of thetransistors 620 within thermometer portion 610 can be replaced with asingle transistor controlled by a digital-to-analog converter. Moreover,some components are shown directly connected to one another while othersare shown connected via intermediate components. In each instance themethod of interconnection, or “coupling,” establishes some desiredelectrical communication between two or more circuit nodes, orterminals. Such coupling may often be accomplished using a number ofcircuit configurations, as will be understood by those of skill in theart. Therefore, the spirit and scope of the appended claims should notbe limited to the foregoing description.

1. A circuit comprising: a. a first reference voltage node adapted toprovide a first reference voltage; b. a reference resistor having: i. afirst reference-resistor terminal coupled to the first reference voltagenode; and ii. a second reference-resistor terminal; c. a secondreference voltage node adapted to provide a second reference voltage; d.an on-die termination (ODT) resistor having: i. a first ODT-resistorterminal; and ii. a second ODT-resistor terminal; e. a multiplexerhaving: i. a first multiplexer input terminal coupled to the secondreference voltage node; ii. a second multiplexer input terminal coupledto the second ODT-resistor terminal; and iii. a multiplexer outputterminal; and f. a comparator having: i. a first comparator inputterminal coupled to the second reference-resistor terminal; ii. a secondcomparator input terminal coupled to the multiplexer output terminal;and iii. a comparator output terminal.
 2. The circuit of claim 1,further comprising a current source having: a. a first current terminalcoupled to the second reference-resistor terminal and adapted to receivea first reference current of a first-reference-current magnitude; b. asecond current terminal coupled to the second-ODT resistor terminal andadapted to receive a second reference current of asecond-reference-current magnitude proportional to the firstreference-current magnitude; and c. at least one current-controlterminal adapted to receive a current-control signal; d. wherein thecurrent source adjusts the first and second magnitudes in response tochanges in the current-control signal.
 3. The circuit of claim 2,further comprising a feedback loop coupled between the comparator outputterminal and the current-control terminal.
 4. The circuit of claim 3,wherein the feedback loop includes a current-control register.
 5. Thecircuit of claim 2, further comprising a first load transistor coupledin series with the first current terminal and a second load transistorcoupled in series with the second current terminal.
 6. The circuit ofclaim 5, further comprising a current-shorting transistor coupledbetween the first and second current terminals.
 7. The circuit of claim5, further comprising a second current-shorting transistor coupledbetween the first and second current terminals via the first and secondload transistors.
 8. The circuit of claim 7, wherein the first and thirdreference voltages are substantially equal voltages.
 9. The circuit ofclaim 1, wherein the ODT resistor includes at least one ODT-resistorcontrol terminal, the circuit further comprising a feedback loop coupledbetween the comparator output terminal and the at least one ODT-resistorcontrol terminal.
 10. The circuit of claim 9, wherein the feedback loopincludes an ODT-control register.
 11. The circuit of claim 1, whereinthe comparator alternately compares: a. a reference-resistor voltage onthe second reference-resistor terminal to the second reference voltage;and b. the reference-resistor voltage on the second reference-resistorterminal to an ODT voltage on the second ODT-resistor terminal.
 12. Thecircuit of claim 1, wherein the first and second magnitudes aresubstantially equal.
 13. The circuit of claim 1, wherein the ODTresistor, comparator, and current source are portions of a monolithicintegrated device, and wherein the reference resistor is external to themonolithic integrated device.
 14. The circuit of claim 1, furthercomprising a third reference voltage node coupled to the firstODT-resistor terminal.
 15. The circuit of claim 1, further comprising:a. a plurality of signal nodes receiving input signals; b. a pluralityof signal-node ODT resistors, at least one ODT termination resistorcoupled to each of the signal nodes, each signal-node ODT resistorincluding a resistor-calibration port; and c. an ODT calibration buscoupled between the comparator output terminal and theresistor-calibration ports of the signal-node ODT resistors.
 16. Thecircuit of claim 1, further comprising: a. a plurality of signal nodesreceiving input signals; b. a plurality of signal-node current sources,at least one signal-node current source coupled to each of the signalnodes, each signal-node current source including a current-sourcecalibration port; and c. a current-source calibration bus coupledbetween the comparator output terminal and the current-sourcecalibration ports of the signal-node current sources.
 17. A method ofcalibrating an on-die termination (ODT) resistor on anintegrated-circuit, the method comprising: a. drawing a referencecurrent through a reference resistor having a first reference-resistorterminal, a second reference-resistor terminal, and exhibiting areference resistance between the first reference-resistor terminal andthe second reference-resistor terminal, the reference resistor receivinga first reference voltage on the first reference-resistor terminal andproviding a reference-resistor voltage on the second reference-resistorterminal; b. drawing an ODT current proportional to the referencecurrent through the ODT resistor, the ODT resister having a first ODTterminal and a second ODT terminal and exhibiting an ODT resistance, theODT resistor receiving a second reference voltage on the firstODT-resistor terminal and providing an ODT-resistor voltage on thesecond ODT-resistor terminal; c. comparing, with an on-die comparator,the reference-resistor voltage with the ODT-resistor voltage to producea current-correction signal; and d. adjusting the ODT current inresponse to the current-correction signal.
 18. The method of claim 17,further comprising: e. comparing, with the on-die comparator, thereference-resistor voltage with the ODT-resistor voltage to produce anODT-resistor-correction signal; and f. adjusting the ODT resistor inresponse to the current-correction signal.
 19. The method of claim 18,further comprising repeating (a) through (d) of claim 17, to readjustthe ODT current.
 20. The method of claim 19, further comprisingconnecting the second reference-resistor terminal and the secondODT-resistor terminal before readjusting the ODT current.
 21. The methodof claim 18, wherein the ODT current is adjusted before adjusting theODT resistor.
 22. The method of claim 18, wherein the ODT current isadjusted before and after adjusting the ODT resistor.
 23. A datacommunication system comprising: a. a first integrated-circuitsupporting: i. a driver circuit having: (1) a driver input nodereceiving an input data stream; (2) a driver output node providing anoutput data stream derived from the input data stream; (3) adriver-current control port receiving a current-control signal; ii. atermination element having: (1) a termination-element terminal coupledto the driver output node; and (2) a ODT-calibration port receiving anODT-calibration signal; and iii. an ODT control system having aresistance-calibration port coupled to the ODT-calibration port, and acurrent-calibration port coupled to the driver-current control port; b.a signal transmission line having a first and second transmission-linenodes, the first transmission-line node coupled to the driver outputnode; and c. a second integrated-circuit supporting a receiver circuit,the receiver circuit having a receiver input node coupled to the secondtransmission-line node and receiving the output data stream.
 24. Thesystem of claim 23, further comprising a reference-voltage sourceexternal to the first and second integrated-circuits and providing areference voltage to the ODT control system.
 25. The system of claim 23,further comprising a reference resistor external to the first and secondintegrated-circuits and providing a reference resistance, the referenceresistor coupled between a reference voltage node and the ODT controlsystem.
 26. A digitally controlled termination element extending betweena reference-voltage node and a signal transmission line, the terminationelement comprising: a. an N-line ODT control bus receiving an N-bit ODToffset count; b. a first binary-weighted resistive portion having aplurality of parallel resistive paths extending between thereference-voltage node and the signal transmission line, each resistivepath including a transistor coupled in series with at least one resistorand having a control terminal coupled to one of the N lines of the ODTcontrol bus; and c. a second resistive portion coupled in parallel withthe first resistive portion between the reference-voltage node and thetransmission line, wherein the second resistive portion exhibits atleast three levels of series resistance.
 27. The termination element ofclaim 26, wherein the second resistive portion includes a secondtransistor coupled between the reference-voltage node and thetransmission line, the second transistor having a control terminalcoupled to another of the N lines of the ODT control bus.
 28. Thetermination element of claim 26, wherein the second resistive portionincludes a voltage divider, the voltage divider having a plurality ofvoltage-divider resistors coupled in series between thereference-voltage node an the transmission line.
 29. The terminationelement of claim 28, wherein the voltage divider further comprises aplurality of voltage-divider transistors, each voltage-dividertransistor coupled in parallel with a respective one of thevoltage-divider resistors.
 30. The termination element of claim 29,wherein each voltage-divider transistor includes a control terminalcoupled to one of the N lines of the ODT control port.
 31. Thetermination element of claim 26, wherein the second portion includes anintegrator.
 32. The termination element of claim 31, wherein the secondportion includes a second transistor coupled between thereference-voltage node and the transmission line and having a secondcontrol terminal coupled to the integrator.